Cromwell dissolves the Rump Parliament and establishes himself as lord protector of England in 1653.
Photograph: Hulton Archive/Getty Images

The controversial inclusion of Oliver Cromwell in the Penguin Monarchs series will doubtless elicit a few tuts of disapproval from royalists. What is Cromwell doing – as David Horspool concedes, “even in square brackets” – in a list of kings and queens that runs from Athelstan to Elizabeth II? Monarchy is, however, a slippery concept and not to be crudely equated with kingly titles or the blue blood that Cromwell conspicuously lacked. Strictly speaking, Horspool reminds pedants, monarchy means a sole ruler, not necessarily the holder of a royal title. Today, the Trump presidency more closely approximates to monarchical rule than the highly circumscribed position of the British Queen.

The obvious contrast between monarchies and republics is less certain than received wisdom allows, and especially so in the case of Cromwell. Notwithstanding the “certitude” of Cromwell’s Puritanism, he was strangely indifferent to political forms. He had no vision of England as a republic, and took a title – lord protector – that was well-established in English history, as an office for regents during royal minorities. There are further wrinkles. Although Cromwell declined a formal offer of kingship in 1657, on his death in 1658 his son Richard succeeded him. “Tumbledown Dick” had none of his father’s mettle, and in 1659 abdicated under pressure from the army. The rule of non-royal Cromwellian “monarchs” gave way to a shuffled sequence of short-term parliamentary expedients supervised by jostling generals, and eventually – through lack of a widely accepted alternative – to the return in 1660 of the Stuart dynasty under Charles II.

Horspool’s central question concerns Cromwell’s ascent to monarchical power. His background in the East Anglian gentry was solid and respectable, and he did not stand out from his contemporaries either through birth or evident talent. It was the breakdown in relations during the early 1640s between Charles I and the Long Parliament, in which Cromwell sat as an MP for Cambridge, that gave him his chance to escape the rut of conventional expectations. In the early stages of the English civil wars, Cromwell, a valiant cavalryman and capable recruiter, advanced briskly through the ranks of the parliamentary army. He was promoted in early 1643 from captain to colonel, and again at the end of that year to lieutenant-general.

At the core of Horspool’s accessible and compelling account of the civil wars is the central dilemma laid out by the parliamentarian leader, the Earl of Manchester: “If we beat the King 99 times, yet he is King still.” Never mind the problem of winning the civil war, how was such a war to be brought to a convincing conclusion? Could the King – even in defeat – be trusted to observe any agreed settlement, especially in the longer term? Horspool rightly devotes as much space to the period of Charles I’s captivity – variously in the hands of the Scots, parliament and the army – from 1646-9 as to the battles of the main phase of the civil war between 1642 and 1646 that had brought him to defeat.

On top of the vexing issue of how far the victors could trust Charles I, the victorious Long Parliament was itself divided by religious questions into two main groups: the Presbyterians, who favoured a reformed national church, and independents, including Cromwell, who were committed to a decentralised arrangement along congregational lines. The Scots, who had also revolted against Charles I, favoured Presbyterianism. Divisions also emerged between the army and their supposed masters in Parliament. Indeed, Cromwell had been specially exempted from the provisions of the self-denying ordinance of 1645, a measure – preventing MPs and peers from commanding parliament’s forces – that a hypocritical Cromwell had strongly supported.

Tim Roth as Oliver Cromwell, left, and Dougray Scott as Thomas Fairfax in the 2003 film
To Kill a King. Photograph: Laurie Sparham/Pathé

Windfalls of this sort had a strange habit of landing in his lap. Was Cromwell, Horspool wonders, the invisible hand behind a series of remarkable coups? Who arranged for Cornet George Joyce – a low-ranking officer at the head of a troop of horse – to seize the King from his captivity at the hands of parliament in Holmby House and transfer him to the care of the army? How did Charles manage to abscond to the Isle of Wight (then under the governorship of Cromwell’s cousin Robert Hammond)? Charles I, the notional loser in the first civil war, was able to exploit tensions among the victors, and a short-lived second civil war ensued in 1648, in which the Scots were allied with the King. He was defeated once more, but the country was no further forward. How could a stable settlement be achieved when Presbyterian moderates in the Long Parliament favoured a compromise with the utterly untrustworthy King? The fatal step towards Charles I’s execution on 30 January 1649 could only be taken once parliament was reduced to a rump purged of its moderates. Was a more senior military figure behind Colonel Thomas Pride’s purge of Presbyterian moderates from the Long Parliament in late 1648? Horspool identifies Cromwell as an absent presence at many such moments, whether through guile, innocence, or because he was such a “champion ditherer”.

Dithering was the hallmark of Cromwellian government between 1649 and 1658

Such dithering was the hallmark of Cromwellian government between 1649 and 1658. At first, between 1649 and 1653, the Rump Parliament ruled, with Cromwell sitting on the council of state. He was officially second in command of the army after Fairfax, a Presbyterian general. The English civil war was over, but there were battles to be won in Ireland and Scotland. Cromwell – whose name still produces shudders of revulsion across the Irish Sea – intervened genocidally in Ireland to suppress Catholic holdouts at Drogheda and Wexford. Whereas England in May 1649 had declared itself a Commonwealth, without king or lords, the Scots had already proclaimed Charles II as their king. Cromwell defeated the Scots at Dunbar and Worcester – at last assuming the highest of commands, because Fairfax was reluctant to wage war on his fellow Presbyterians.

Back in England, however, the Rump was unable to command a broad base of support. At this late, irretrievable stage, Horspool surmises, Cromwell was among those anxious politicians who “could still contemplate a monarchy, and a Stuart sitting upon the throne”. In 1653 Cromwell lost patience with the Rump and instituted a new nominated assembly, known to historians as the Barebones Parliament. It did not last long, and by the end of 1653 Cromwell had accepted an invitation to become lord protector. A further run of experiments ensued, as Cromwell – hypocrite-ingénu and hard man – attempted, with limited success, to form a government on sound, widely acceptable foundations. Cromwell’s two Protectorate Parliaments were punctuated by an exasperated period of junta-like rule by major-generals. But legitimacy – shorn of the foliage of tradition – proved elusive, as we know all too well. Although today we live in a kind of “crowned republic”, republicanism remains taboo in mainstream politics; and the House of Lords has just celebrated more than a century on seemingly borrowed time.

• Colin Kidd’s books include British Identities Before Nationalism and The World of Mr Casaubon. Oliver Cromwell is published by Allen Lane. To order a copy for £11.04 (RRP £12.99) go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99.